A Field to Himself
In early 1941, the astrophysicist Ralph B. Baldwin (1912–2010) was an instructor of astronomy at Northwestern University and on the verge of becoming a father. In order to support his soon-to-be-enlarged family, Baldwin began to moonlight by giving lectures at Chicago’s Adler Planetarium, for four dollars each. When he arrived early, he would wander the halls, examining the Adler’s exhibits and especially its superb photographs of the Moon. One in particular caught Baldwin’s eye: it showed a set of “long valleys with raised rims” that he had never seen mentioned in the literature. He noticed that these valleys “pointed backward toward Mare Imbrium.”1
His curiosity aroused, Baldwin searched the library of the Dearborn Observatory for an explanation of the strange valleys. Finally he found a paper that attributed them to “the nearly tangential impact of a swarm of huge meteorites,” a suggestion that did not strike him as “viable.” As he continued to peer at the photographs, Baldwin began to reconsider the role of meteorite impact on the Moon: “Could it be that Mare Imbrium was the moon’s largest crater and the valleys were splash craters from debris ejected at the birth of the huge pit? Were there others like Imbrium ?” Answering his own question, he exclaimed: “There were! Crisium, Nectaris, Humorum, Serenitatis and Humboldtianum.” Baldwin concluded that “the circular maria had been produced by gigantic explosions—only the impact of giant meteorites could supply the requisite energy.”2
A few weeks after coming to this startling conclusion, the unknown Baldwin attended a colloquium that drew several eminent astronomers. He soon discovered that none of them knew or cared much about the Moon—his few weeks of study had left him “knowing far more about the Moon than any of them.” Baldwin realized that “nobody else was actively mining this lode” and that he had “a significant field almost to himself” (369–370). He wrote up his ideas, titling the paper “The Meteoritic Origin of Lunar Craters,” and submitted it to leading astronomy journals and to the Annals of the Dearborn Observatory. All rejected the article. Finally Popular Astronomy, where T. J. J. See had often published, printed Baldwin’s maiden paper.3 Irritated by the cavalier rejection of his ideas by the astronomy establishment, Baldwin set out to prove his critics wrong. He would show that the Moon is a worthy subject for study and that his theories were sound.
Baldwin’s first major contribution to lunar geology was his 1949 book The Face of the Moon. Like Dietz, Baldwin was unaware of Wegener’s pamphlet on the subject, but all three came to the same conclusions. The book included a logarithmic graph of depth versus diameter for 329 lunar craters. The data points fell on a smooth curve: over a range of diameters of more than 150 times, the wider the crater the deeper. The clincher came when he plotted on the graph the dimensions of explosion pits from wartime mortar and bombshells. Though smaller than the lunar craters, these known explosion craters fell right on the lunar curve. Baldwin concluded that “the only reasonable interpretation of this curve is that the craters of the Moon, vast and small, form a continuous sequence of explosion pits, each having been dug by a single blast. No available source of energy is known other than that carried by meteorites.”4
Baldwin summed up: “To claim that the Moon’s craters are volcanic is tantamount to postulating an entirely new, entirely hypothetical mode of origin and to fly in the face of the fact that a known process is completely able to explain the vast majority of observed lunar features” (146). But in 1949, hardly any scientists were interested in the Moon and its craters. To them, the matter appeared long settled: lunar craters are volcanic, and, moreover, so are all putative terrestrial impact craters. Fortunately, there were a few who did not see it that way.
Shock Waves
One who read The Face of the Moon was the Nobel laureate and chemist Harold Urey (1893–1981). The book so inspired Urey that he made the origin of the Moon and planets the focus of his life’s work. Another was Gene Shoemaker. When Baldwin’s book appeared, Urey was already famous; at age twenty-one, Shoemaker was not famous—yet. Like Harry Hess, Urey was often grandly wrong. Again like Hess, when Urey was wrong he promptly admitted it and modified his ideas to incorporate the new evidence. If anything, Urey may have been too ready to change his mind. But how much better that than a refusal to debate or to admit new evidence.
Urey read Baldwin’s book on a Canadian train trip. On his return, he lined his office with lunar photographs and became consumed with the Moon. He was not particularly interested in the origin of lunar craters, having no reason not to accept Baldwin’s documented conclusion that they were caused by impact. Urey’s interest lay in the possibility that the Moon could tell us much about the early history of the Earth.
Scientists had long known that because of the tug of the Earth’s gravity, the Moon bulges slightly toward the Earth. Since over geologic time the protrusion has not receded, Urey speculated that it must have formed early in the Moon’s history and been frozen in. Therefore the Moon must not have isostasy—its interior must be cold and rigid. Such a moon was likely to be primordial, Urey thought, and thus represent a fossil from the earliest history of the solar system. By studying the Moon, we could learn about the starting conditions of all the planets and moons, including our home planet. Urey published his ideas in a 1952 book called The Planets. It was widely read and so persuasive that it caused many physical scientists to change the direction of their research.5
Harold Urey, given his age, stature, and university tenure, was free to let his mind roam wherever he wished, even to the faraway Moon. Shoemaker’s remarkable life has been well recorded by his friend and colleague David Levy.6 In the 1950s, his career was just getting off the ground.7 Shoemaker worked for the USGS, which still lay in thrall to G. K. Gilbert. For half a century, the Survey had discouraged its employees from studying terrestrial craters, much less lunar ones. As a way to pursue his interest in the Moon while acceding to the boundaries established by the Survey, Shoemaker hit on the idea of studying the Hopi Buttes, which he thought resembled volcanic necks and chains of craters on the Moon. The inventive Shoemaker justified this work by convincing his superiors that, as the volcanic material that was to become the buttes had risen to the surface, it might have gathered up uranium deposits, a major focus of the USGS at the time. From there Shoemaker moved on to study actual craters produced not by meteorites or volcanoes but by underground nuclear explosions at the Nevada Test site. This naturally led him to nearby Meteor Crater.
Having no evidence to the contrary, Shoemaker started out assuming that Gilbert had been right to consider Meteor Crater volcanic. But by 1959, Shoemaker had laid to rest the notion that Meteor Crater is of terrestrial origin. He showed that meteorite impact generates two shock waves that interact in complex fashion to destroy the incoming meteorite and excavate the resulting cavity. When the meteorite, traveling at cosmic velocities, first makes contact with the surface of a solid body, one shock wave moves back into the meteorite to engulf and blast it to pieces. Ahead of the incoming projectile, another shock wave races down into the incipient crater, compressing the target rocks, then ejecting them at great speed. Fragments of target rock thrown out on low trajectories land in the reverse order in which they departed, stacking upside down relative to their original layering. Much of the nearby ejecta slumps back into the crater. The process produces an explosion like that of a bomb, and it leaves a circular crater regardless of the meteorite’s angle of impact, something that Gilbert and the other pioneers had no way of knowing.
Shoemaker’s detailed geological mapping showed that the rocks around Meteor Crater perfectly manifest this theoretical process. The coup de grace to cryptovolcanism was his discovery of two rare minerals derived from quartz at Meteor Crater, one named coesite after its synthesizer. Scientists had produced the two minerals in extreme high-pressure laboratory experiments, but they had never been found in nature.
Bucher’s Last Field Trip
In Walter Bucher’s paper at the second of the two 1964 conferences, he argued that three of the largest cryptovolcanic structures are associated with known geologic structures and thus cannot have resulted from random events.8 One of the three “so-called meteorite scars” on which Bucher focused, the Wells Creek Basin in Tennessee, he said was “aligned along an anticlinal axis.” Another, the Ries structure in Germany, was related to “the crest of a major downfold.” The third, the Vredefort Dome in South Africa, lay “in line with four other circular domes.” This type of evidence may be suggestive, but it is hardly conclusive. Countless anticlinal folds and downfolds exist, and a meteorite landing at random might well hit near one of them.
Because some cryptovolcanic structures have shatter cones and coesite, yet according to Bucher impact could not have caused the structures, he had to conclude that the two putative markers must “not be accepted as sufficient evidence for impact from above.”9
On May 2, 1964, just before the conference convened, at Shoe maker’s invitation Bucher visited Meteor Crater. A former Bucher student, Wolfgang Elston, was along to provide a first-person account.10 “Gene’s meticulous structural mapping convinced Bucher of the realities of the overturned rim and outward thrusts,” Elston wrote, continuing, “as we left, [Bucher] conceded Meteor Crater, ‘but the Ries, that’s different.’”
The “Ries” is a large circular geologic structure in Germany long believed by Bucher and German geologists to be of volcanic origin.11 In July 1960, just after the death of his father, Shoemaker, his wife Carolyn, and his mother were on their way to Copenhagen, where Gene would attend a conference. Deciding to do some “geological sightseeing” on the way, the family stopped by the town of Nördlingen, Bavaria, to visit the nearby twenty-four-kilometer-diameter Rieskessel: the Ries “bowl” or “kettle.” One feature of the Ries was a peculiar glassy rock named suevite. Shoemaker recognized on sight that it was a solidified impact melt-rock. He collected samples and sent them off to Ed Chao, the USGS colleague who had identified high-pressure coesite in rocks from Meteor Crater. While in Nördlingen mailing the package, the family decided to visit St. George’s Cathedral, which stood above the small town. Shoemaker whipped out his ever-present hand lens and spotted suevite in the building stone of the cathedral. It would later be found to contain microscopic diamonds formed when the meteorite struck a bed of rock rich in graphite.12 Chao promptly identified coesite in the suevite. Four years later, departing Meteor Crater, Bucher would still deny that impact had formed the Ries crater.
Elston subtitled his account of the field trip: “Walter Bucher’s Last Field Trip and Conversion to the Impact Origin of Meteor Crater: A Tribute to an Open Mind.” This is a jarring epitaph for Bucher, who went to his grave refusing to accept continental drift, paleomagnetism, and, except for Meteor Crater, meteorite impact.13
As a few scientists began to entertain the possibility that impact might have created the craters of the Moon, attention turned to possible terrestrial analogues. In The Face of the Moon, Baldwin listed twelve likely impact craters, excluding Meteor Crater but including the Rieskessel and the six that Bucher had described in 1933. Nevertheless, most of the speakers at the 1964 conference on “Geological Problems in Lunar Research” continued to argue that the Moon’s surface features are volcanic and that there are no terrestrial impact craters. Even proponents of impact, like Baldwin, Shoemaker, and Urey, at the time accepted that many, if not most, of the smaller lunar features are volcanic.
After hundreds of years of viewing the Moon from a distance that made any theory safe from a reality check, by the mid-1960s the effective distance was about to close to zero. Surely, scientists would now be able to translate the hieroglyphics of lunar geology. Surely, the Rosetta Stone of the solar system would now unveil her long-held secrets.